The present invention generally relates to a solar cell. More particularly, the invention concerns a solar cell of an amorphous silicon material in which a semiconductor thin film of a superlattice structure is used and which exhibits a high photoelectric conversion efficiency.
Owing to remarkable progress in the field of crystal growth technology and amorphous thin film growth technology, it is now possible to realize a so-called ultra-thin film having a thickness (on the order of 10 to 100 .ANG.) substantially equivalent to the electron wavelength (de Broglie wavelength). In such ultra-thin films, two-dimensional electric conduction which cannot be seen in a bulk semiconductor occurs. By making use of this feature, various applications of the ultra-thin film are reported (reference may be made, for example, to Hirose et al.'s article titled "Semiconductor Superlattice and Photoelectric Process" contained in "Data No. 425 of 111-th Investigation Meeting of 125 Committee of Japan Learning and Study Advancement Society", pp. 13-18). For practical applications, it has been proposed to utilize superlattice structures by doping the superlattice structure with various types of impurities such as an impurity for controlling conductivity type, an impurity for forming a deep level, an impurity for producing a luminescent center, and others.
However, no consideration has heretofore been given to the impurity distribution and optimization thereof within the ultra-thin film itself as well as in the layers of the superlattice structure. In practice, investigations do not go beyond the control of potential distribution in the thickness direction, i.e. control of the depletion layer and quantization levels within a potential well.
In the hitherto known solar cells of amorphous silicon type semiconductors, it is necessary to realize the p-type layer in a thickness smaller than 100 .ANG. in order to obtain sufficiently high short-circuit current, thereby giving rise to the problem that the open-circuit voltage is low when compared with a thick film solar cell. In the case of the thick film cell, a sufficiently high short-circuit current can be generated by widening the optical gap by increasing the amount of carbon added to the silicon, which, however, causes the doping effect of boron to be lowered to decrease the photoconductivity, whereby the photoelectric conversion efficiency is correspondingly reduced. In other words, great difficulty has been encountered in the attempts to increase the short-circuit current, the photoelectric conversion efficiency, and the open-circuit voltage. Parenthetically, in the hitherto known pin-type amorphous silicon solar cell, the short-circuit current density is typically 16 mA/cm.sup.2, the open-circuit voltage is typically 0.85 volts, and the photoelectric conversion efficiency is typically 9.5%.